bt2: remove unused variable in UserMessageIteratorSeekNsFromOriginTestCase.test_seek_...

bt2: remove unused variable in UserMessageIteratorSeekNsFromOriginTestCase.test_seek_ns_from_origin

Flake8 reports:

    ./tests/bindings/python/bt2/test_message_iterator.py:790:9: F841 local variable 'msg' is assigned to but never used

Reported-by: flake8
Change-Id: I4be9bf2d1a86e4c5d538fd1d1461f6e2bbf2b763
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/2092
Reviewed-by: Francis Deslauriers <francis.deslauriers@efficios.com>